The administration, campaigning hard to push through Congress its proposed sale of sophisticated radar planes to Saudi Arabia, staged a special press briefing yesterday to clarify what it called "misunderstandings" about the planes' capabilities and whether they pose a threat to Israel's security.

The White House briefing, conducted by President Reagan's national security affairs adviser, Richard V. Allen, put so much stress on what the Airborne Warning and Control System (AWACS) planes cannot do that it caused some tongue-in-cheek murmuring in the audience about why the Saudis would want to spend $5.8 billion for the five planes.

With the aid of an Air Force briefing officer, Col. Robert Lilac, Allen spent an hour describing the plane's functions as a "flying radar platform" whose purpose would be quick detection of aircraft flying into Saudi air space.

Allen persistently sidestepped questions about the central issue in the administration's uphill fight to prevent Congress from vetoing the proposed sale.

That involves whether sufficient safeguards are built into the deal to ensure that the AWACS planes are not used to spy on air traffic within Israel and are protected from falling into the hands of U.S. enemies.

Allen and Lilac said repeatedly that the planes are intended to help guard against attacks on the Saudi oil fields by Soviet proxies and other potential foes. They insisted that offensive use of the AWACS against Israel is very unlikely because the planes cannot see into Israeli territory without being moved so close that they would be vulnerable to Israeli fighters.

Allen refused to answer questions about details of safeguards contained in the still-secret transfer arrangements worked out with the Saudis.

Asked about repeated statements by Saudi leaders that they consider Israel their main enemy, Allen said that despite such public statements the administration is convinced, on the basis of private talks with the Saudis, that they consider the Soviet Union the main threat to their security.

In another development, former secretary of state Cyrus R. Vance, who led the successful 1978 fight by the Carter administration to sell 62 F15 jet fighter-bombers to Saudi Arabia, sent a letter to Sen. Alan Cranston (D-Calif.) opposing the new sale, which also would give the Saudis enhanced fuel capacity for the Fl5s, air-to-air Sidewinder missiles and aerial refueling equipment.

Four former ambassadors to Saudi Arabia--Robert G. Neumann, John C. West, James E. Akins and Parker T. Hart--called for approval of the sale and warned that rejection could lead the Saudis to change their moderate policy on oil sales to the West.

Similarly, former president Gerald R. Ford said he is lobbying actively for congressional approval of the sale, and he criticized Israeli Prime Minister Menachem Begin's public opposition as "trying to interject himself into a decision of the United States that ought to be best left to President Reagan and our country."

Former secretary of state Henry A. Kissinger said that it was a mistake to offer the AWACS to the Saudis in the first place, but that it would be a worse mistake not to go through with the deal now struck.